A technique referred to as "Q-switching" generates short laser pulses by placing an optical switch in the laser cavity. Q-switching is described by U.S. Pat. Nos. 3,613,024 and 3,928,815, issued to Geusic and to Hellwarth, respectively, and incorporated herein by reference. In Q-switched lasers, an energy source supplies electrical or optical excitation to or "pumps" the laser medium. A user triggers the optical switch from a high loss condition to a low loss or "high Q" condition, allowing a laser pulse to build up and issue from the optical cavity. During the period of pumping, energy is simultaneously absorbed from pumping and dissipated through spontaneous emission and non-radiative transfer. Energy dissipation from the upper laser level occurs at a rate approximately proportional to the upper laser level population, and inversely proportional to a material-dependent parameter, the effective decay lifetime. This decay lifetime is affected by many physical parameters of the material; to name a few, the material composition, the pressure, the temperature, and the density of other atoms, ions, or molecules in different quantum states that can interact with the upper level population. The energy in each laser pulse depends on the pumping rate, the time between pulses, and the population density distribution of the laser medium after the last pulse, if one occurred.
A laser's pumping can be either continuous (cw) or pulsed. Here, "cw pumping" refers to pumping by a source that may be, but need not be, operated at constant power over time. The cw pump may be modulated to different levels, as distinguished from pulsed pumping which exclusively operates in brief, discrete time intervals. A cw-pumped Q-switched laser can generally operate either with pulsed output, or with cw output, if the Q-switch is suitably driven. A feature of cw pumping is that the user can Q-switch the laser at nearly any time and the laser emits a pulse, provided there is sufficient time to absorb pump energy; there is no need to wait for a burst of pump energy. However, with continuous pumping, the performance parameters of a particular pulse vary depending upon the time duration of laser pumping between pulses.
As described by Wagner and Lengyel, Jour. Appl. Phys., vol. 34 (1963) 2040- 2046, the stored energy and gain in the laser, along with the length of the laser optical cavity, determine the minimum attainable pulse width. Increasing the gain allows reduced pulse widths, and the gain is proportional to the stored energy. According to Chesler, Karr and Geusic, Proc. I.E.E.E., vol. 58 (1970) pp. 1899-1914, with continuous pumping the stored energy, E.sub.s, varies approximately according to EQU E.sub.S =E.sub.0 [1-exp (-t.sub.S /T)], (1)
where t.sub.s is the duration of energy storage, T is the fluorescence decay lifetime of the laser material, and E.sub.0 is the maximum energy stored. Repetition rates much greater than 1/T are considered "high" in this context, and repetition rates much less than 1/T are "low". At low pulse repetition rates, the stored energy, E.sub.s, approaches the laser's maximum storable energy, E.sub.0, and the pulse width approaches its corresponding minimum. Consequently, the peak pulse power asymptotically approaches a maximum as the pulse repetition rate is reduced. If a burst of many pulses at high repetition rate is emitted, the laser generates the first pulse of the burst after an extended energy storage time, and the first pulse is therefore more energetic than the subsequent pulses. The energy delivered by this first pulse can be large enough to be catastrophic in many applications, from medical to microelectronic micromachining.
Three desirable characteristics of a pulsed laser are that: (1) it is stable over time; (2) it has a similar pulse energy regardless of the time duration between pulses; and (3) it is not subject to optical damage. In laser design, these characteristics are balanced against other desires, such as high energy pulses in some applications. Furthermore, for some applications, such as high repetition rate materials processing or high data rate optical probing, the laser must work reliably at multi-kilohertz repetition rates. The laser need not have increased pulse energy or reduced pulse width at lower repetition rates. In fact, it is often desirable that performance remain unchanged from an application-determined, multi-kilohertz rate down to lower rates. It is generally unacceptable if the laser cannot perform reliably at low repetition rates or, worse yet, if the more energetic laser pulses at low repetition rates damage one or more laser optical components.
A particular area where it is advantageous to maintain an upper limit on the pulse energy at low repetition rates is with lasers optimized for short pulses at high repetition rate operation. Often it is desirable to maintain nearly constant pulse energy, pulse width, pulse power, and pulse timing delay as the repetition rate is varied from 0 Hz to many kilohertz. For those lasers operating on relatively low gain transitions, such as the 1.3 .mu.m lines of neodymium-doped solid-state lasers Nd:YAG and Nd:YLF, restricting the power of pulses at low repetition rates could prevent optical damage. Because the 1.3 .mu.m wavelength is near a dispersion minimum in optical fiber materials, lasers that emit at 1.3 .mu.m are of technological importance and are important sources for fiber optic sensors and communications. Lasers of wavelength 1.3 .mu.m have been operated successfully with pulse widths shorter than 10 nsec at 10 kHz. Users may run these lasers at lower repetition rates as well. Unfortunately, the low stimulated emission cross-section of these lines means that high intra-cavity fluences are required to efficiently extract energy. Low repetition rate operation can cause single-pulse optical damage to intra-cavity elements if the energetic pulses are allowed to develop. If the Q-switch in the laser system cannot produce enough loss to prevent laser emission at low repetition rates, although the laser may not be damaged, its performance at low repetition rates will be irregular and useless for many applications.
Many control systems can help equalize the energy of pulses from a laser. One solution is a closed loop control system that measures a parameter of each pulse, such as energy, and corrects the pumping level accordingly to stabilize that parameter at a fixed level for subsequent pulses. Such a system bases its feedback signals on the pulse history and cannot look ahead. If the repetition rate changes rapidly, there are transitional periods of poor output control that can produce catastrophic optical damage. An example of such a transition in repetition rates is the initial power-up of a laser: The laser goes from 0 Hz to an operational frequency of possibly tens of kilohertz, and the first pulse issued will be very large. Similarly, if the user triggers pulses at random intervals, difficulties will arise.
The first energetic pulse can be suppressed if the pump source is extinguished at the start of a pulse burst. This technique is described in U.S. Pat. No. 4,337,442, issued to Mauck for a laser pulse modulation system, where energy variation in pulses is reduced. However, this is done by elimination of pulses, and the remaining issued pulses are unavoidably delayed from the trigger signal that initiates the pulse burst. Another limitation of the technique in certain applications is that, if the user desires a single pulse, the system may simply eliminate it, believing it to be the first pulse of a burst.
Another potentially useful pulse control method requires a time delay between the arrival of a trigger signal and the emission of a pulse. Then the laser control system knows the time that will be allotted for pumping, and can anticipate the required pump level to produce a desired pulse energy. This approach introduces long delays between trigger and each issued pulse, which is unacceptable in some applications. Yet another approach is to monitor spontaneous emission fluorescence from the laser medium as a measure of stored energy. A feedback system controls the pump power in order to equalize fluorescence level for all pulses. Laser fluorescence can be a useful measure of stored energy in some applications, but it is not a perfect predictor of the laser gain and pulse widths because the fluorescence is not restricted to the excited population strictly within the mode volume of the laser. Another approach intentionally uses a Q-switch with inadequate Q spoiling, preventing sufficient energy build-up so that powerful damaging pulses do not occur. However, at low repetition rates, a laser with an inadequate Q-switch will emit light in a periodically interrupted cw output, and the pulse energy stability will be generally poor.
Feedback mechanisms and the like have been used from time to time to control some aspect of laser Q-switching or other excitation. In U.S. Pat. No. 3,258,596, issued to Green, a pulse frequency-modulated injection laser is disclosed in which the repetition rate of a voltage-controlled current switching device is controlled to allow sufficient time for a laser diode switch to recover to generate a new switching signal.
Fenner, in U.S. Pat. No. 3,478,280, discloses pulse-width-modulated laser switching of a semiconductor laser by adjustment of the turn-on delay of the laser. From one perspective, control of the current amplitude applied to the laser allows control of the time required to fill or saturate the photon traps present in the laser material. This, in turn, controllably delays the time at which photon build-up occurs before a laser pulse is emitted. This effect is more significant at room temperature than at lower temperatures. This approach is not directly applicable to Q-switched lasers.
A laser feedback circuit for controlling relaxation oscillation pulses in a solid-state laser is disclosed in U.S. Pat. No. 3,633,124, issued to Danielmeyer. The phase difference between a stable periodic perturbation of a laser pump source and the laser output pulse is externally detected, and a feedback signal is used to restore this phase difference to a desired level. Here, the goal is to produce peak output pulse intensities that are as much as 20 times the average laser power, not to limit the output intensity to a given range for all pulses including start-up.
Paoli et al disclose apparatus for narrowing the pulse width and stabilizing the repetition rate in semiconductor lasers with self-induced pulsing, in U.S. Pat. No. 3,641,459. A small portion of the laser output signal is split off, and the present pulse repetition rate is determined. A portion of the laser output signal is reflected by an external mirror and returned to the active region of the laser to perturb the laser output signals. Pulse narrowing and rep rate stability are achieved when the speed of light, divided by the product of pulse rep rate (frequency) times optical path length from laser end face to mirror, is a ratio of two integers. Repetition rate for laser self-pulsing apparently approaches and stabilizes at a value that produces one of these ratios. This approach does not attempt to limit the amplitudes of the output pulses to a given range.
Method and apparatus for stabilizing the amplitude, pulse width and repetition rate of a Q-switched laser output signal is disclosed by Koechner et al in U.S. Pat. No. 3,747,019. The laser output beam is linearly polarized, and all light above a predetermined amplitude level is removed to provide a sequence of intermediate pulses of approximately uniform amplitude. A central part of each pulse of a sequence of pulses is then passed, using one or more Pockels cells as a shutter, to control the pulse amplitude and pulse width of a sequence of output pulses finally issued by the apparatus. No control of initial excitation of the laser medium or the intra-cavity pulse is exercised here. Consequently, large, potentially damaging pulses may be generated within the laser cavity.
What is needed is a control system for cw-pumped, Q-switched lasers to restrain the pulse energy at low repetition rates, while permitting full energy performance at high repetition rates, without requiring advance information on the arrival times of future trigger signals. Preferably, a laser driven by this control system should be stable over time as the pump components age and/or the pump parameters drift with time, and laser excitation input energy should be controlled in order to improve laser operating efficiency.